Plant-based indicators include canopy temperature and time-temperature threshold methods as well as plant water potential (pressure chambers, “pressure bomb”) methods.  Even better is a combination of multiple methods, such as using crop ET estimates as a primary scheduling method, with soil moisture monitoring to verify that the ET estimates are on track. 

One of the main obstacles to adopting irrigation scheduling is the commitment of extra labor and time to collect the field data and additional management time to interpret and apply the information to irrigation decisions.  Convenient data acquisition, processing and interpretation tools are becoming more widely available and should greatly improve the feasibility and adoption of irrigation scheduling methods.

Irrigation management is part of the overall water management program.  Take full advantage of precipitation.  This means maximizing the benefits of rainfall received during the cropping season, as well as maximizing storage of moisture from off-season precipitation. Where applicable, terracing, furrow diking, contour farming and conservation tillage practices can reduce runoff losses of irrigation and rainfall.

Optimum use of soil moisture storage requires a working knowledge of soil moisture characteristics and the effective crop root zone.  Loam and clay loam soils can have relatively high plant available water storage capacities (1.5  to 2 inches of water per foot of soil depth). Sandy soils often have much lower soil moisture storage capacities (one inch or less of water per foot of soil depth). 

Many agronomic crops can have active roots to depths of 5 or 6 feet; however, many will extract most of their water requirement from the top 1 to 3 feet of soil.  Site-specific soil conditions, including plow pans, caliche layers, excessively dry or excessively wet soil conditions, can limit effective root zone depths. Generally speaking, because plants need both oxygen and water for healthy root systems, roots develop best in moist (neither saturated nor extremely dry) soil. Local USDA Natural Resources Conservation Service and Extension specialists and agents can provide additional information on soil moisture monitoring and soil moisture storage characteristics.             

Soil moisture storage information can be used in applying a Management Allowable Depletion strategy.  Management Allowable Depletion is similar in concept to a minimum balance requirement on a checking account.  If too much water is removed from the soil, the resultant drought stress on the crop can result in a yield loss “penalty.”

For many crops, recommendations call for a “minimum balance” of about 50 percent plant available soil moisture should be retained in the soil to prevent drought stress of the crop.  Therefore, irrigation would be managed to replace depleted soil moisture before the soil water storage drops below the 50 percent plant available water “minimum balance.”  For drought sensitive crops, the “minimum balance” should be more conservative to reduce risk of drought stress.

Deficit management

Another irrigation management strategy is often referred to as deficit irrigation management, or more appropriately “managed deficit irrigation.” Deficit irrigation management often is adopted by default due to insufficient irrigation capacity (such as limited well capacities). 

“Managed deficit irrigation” can be more creatively applied to optimize water allocation, whether optimizing the timing of irrigation application for improved crop response or allocating limited water resources between crops in a rotation, taking into account relative drought tolerances of the crops and relative drought sensitivities (or crop response to water) based upon crop growth/developmental stage.

 For instance, drought stress early in a cotton crop (squaring) likely will have a greater relative impact on lint yield than much later in the season (after first open boll).

Corn is most sensitive to drought stress during the critical period two weeks before and after silking.  Managed deficit can also encourage other desired crop-specific product quality responses, such as sugar content, maturity or ease of harvest. 

A note of caution regarding interpretation of water use efficiency may be in order.  Defining “full irrigation” as providing all the water a crop can use, some (but not all) crops seem to respond well to slightly deficit irrigation.  More severe drought stress will result in yield loss, even if the overall “water use efficiency” may be higher.  Conversely, crop responses to incremental improvements in water management are more obvious in more severely water-limited conditions. 

Some very good irrigation technologies and management tools are available, and, appropriately used, they can net some excellent results.  Effective and efficient irrigation management is crucial to conserve water, reduce irrigation costs, and achieve water use efficiency and crop production goals.

rsmith@farmpress.com